1. Field of the Invention
The present invention relates to an integrated process to produce tertiary alkyl ethers from a C4 stream and a C5-C6 refinery cut, more particularly the upgrading of the isoolefins to useful octane improvers, specifically by the production of methyl tertiary butyl ether (MTBE) from the reaction of isobutene with methanol, tertiary amyl methyl ether (TAME) from the reaction of isoamylenes (iC5=""s) with methanol, and tertiary hexyl methyl ether (THEME) from the reaction of isohexenes with methanol in the presence of an acid cation exchange resin.
2. Related Information
The light naphtha refinery cut is valuable as a gasoline blending stock or as source of isoolefins (iC5=""s and iC6=""s) and to form an ether by reaction with lower alcohols. The Clean Air Act and regulations implementing it require the reformulation of gasoline. Requirements for reformulated gasolines include the reduction of benzene and olefins. The reformulation has resulted in the loss of octane in the straight gasoline. Other requirements are (1) to include a certain amount of xe2x80x9coxygenatesxe2x80x9d, such as methyl tertiary butyl ether (MTBE), TAME, tertiary hexyl methyl ether (THEME), ethyl tertiary butyl ether (ETBE) or ethanol, (2) to reduce the amount of olefins and aromatics in gasoline, and (3) to reduce the vapor pressure (volatility). Refiners have elected to meet these federal mandates by preferably using the ethers. The demand for MTBE in some areas has out stripped the available isobutene. Some refiners are looking to the C5 and C6 cuts which contain suitable isopentene and isohexene for producing TAME and THEME. In addition to improving air quality by adding the ether rather than the raw ethanol, which has a somewhat higher volatility, and removing the olefins, the replacement of the pentenes and hexenes with the ethers results in an increase in the octane rating of the stream.
The C4""s used for feed to an MTBE unit are easily separated from the hydrocarbon source and usually contain iC4=, nC4=""s and some normal isobutane.
The C5""s and C6""s in the feed to a TAME/THEME unit are contained in a single xe2x80x9clight naphthaxe2x80x9d cut which contains everything from C5""s through C8""s and higher. This mixture can easily contain 150 to 200 components and thus identification and separation of the products are difficult. For this reason the TAME/THEME produced from these streams is not generally separated from the heavier components, but all are used directly as octane blending stocks.
Mixed refinery streams often contain a broad spectrum of olefinic compounds. This is especially true of products from either catalytic cracking or thermal cracking processes. Refinery streams are usually separated by fractional distillation, and because they often contain compounds that are very close in boiling points, such separations are not precise. A C5 stream, for instance, may contain C4""s and up to C8""s. These components may be saturated (alkanes), unsaturated (mono-olefins), or poly-unsaturated (diolefins). Additionally, the components may be any or all of the various isomers of the individual compounds.
In most light naphtha cuts the isoamylenes and isohexenes suitable for the production of TAME/THEME are frequently present in small quantities, e.g. less than 15%, whereas there are other C5/C6 olefins isomers and enough dienes and acetylenes to inhibit the etherification process. In the present process the valuable ether production is maximized while removing olefins and other impurities found in the C5 cuts. It is an advantage of the present process that the production of TAME/THEME from C5-C6 cuts is maximized. It is a particular feature of the present invention that the treated components can be reblended with the raffinate of the C5 and C6 cuts to provide an upgraded gasoline blending stream. It is a further advantage of the present invention that the raffinate stream is substantially upgraded by the reduction of olefinic content.
Briefly the present invention comprises the steps of feeding methanol and a stream containing iC5=""s and iC6=""s to a first distillation column reactor containing an acidic catalyst wherein a portion of the iC5=""s and iC6=""s are reacted to produce TAME and THEME. Simultaneously the TAME, THEME and unreacted C6""s are removed as bottoms while unreacted C5""s (including unreacted C5=""s) and methanol are removed as overheads. The unreacted C5""s and methanol are fed along with isobutene and additional methanol to a second distillation column reactor where additional iC5=""s are reacted with methanol to produce additional TAME and the isobutene is reacted to produce MTBE. Unreacted C5""s, TAME and MTBE are removed as bottoms while unreacted C4""s and methanol are removed as overheads.